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Modification and analysis of sensor surfaces

Ellipsometry is a suitable tool for characterization of surface substrates enabling the determination of the optical constants n (refractive index) and k (extinction coefficient). To realize a function of a sensor, the Layer-by-Layer method [1, 2] is a fast and simple possibility to modify nearly each surface substrate.

The in-situ modification of a sensor surface can be investigated by Internal Reflection Ellipsometry IRE. [3, 4] The angle of incidence of the laser light from the backside of the substrate's surface, excludes disturbances due to changes in the bulk solution. The ellipsometric angles Delta und Psi are determined in-situ during coating of a transparent substrate using the polyelectrolytes Polyallylamine Hydrochloride PAH and Polyacrylic acid PAA. Based on a layer stack model the thickness and homogeneity of the organic layer can be determined in sub-nanometer resolution. Atomic Force Microscopy (AFM) enables visualization of the characteristic structure of a few square micrometer area of the polyelectrolyte surfaces.

<b>Schematic of the setup for functionalization of transparent surface substrates via Internal Reflection Ellipsometry.</b>
<b>Thickness of 10 double layers PAH-PAA onto Ta<sub>2</sub>O<sub>5</sub> (mean values of 9 regions of interest ROIs, 10-point-smoothening)
1 µm<sup>2</sup> AFM of the PAA surface with a roughness rms = 11.7 nm.</b>
[1] G. Decher, Fuzzy nanoassemblies: Toward layered polymeric multicomposites. Science, 277 (1997) 1232-1237.
[2] G. Decher and J. Schlenoff, Multilayer Thin Films. 2002: Wiley-VCH Verlag GmbH & Co. KGaA. 515 ff.
[3] K. Büchner, N. Ehrhardt, Brian P. Cahill, C. Hoffmann, Zeitaufgelöste ellipsometrische Untersuchungen des Aufbaus von Polyelektrolyt-Multischichten auf optisch transparenten Oberflächensubstraten mittels Interner Reflexion, 15. Heiligenstädter Kolloquium (Tagungsband), ISBN 978-3-00-032249-5 (2010) 257-264.
[4] K. Büchner, N. Ehrhardt, Brian P. Cahill, C. Hoffmann, Internal reflection ellipsometry for real-time monitoring of polyelectrolyte multilayer growth onto tantalum pentoxide, Thin Solid Films 519 (2011) 6480-6485.
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